The present invention concerns a method of exhaust gas aftertreatment. Methods of exhaust gas aftertreatment are frequently used to comply with the emission limit values of internal combustion engines. A method which is also known from the field of exhaust gas aftertreatment of caloric power plants is regenerative thermal oxidation (RTO) in which unburnt hydrocarbons and other oxidizable exhaust gas constituents are thermally oxidized. In regenerative thermal oxidation the exhaust gas is firstly passed by way of a heat storage means generally comprising ceramic bulk material or honeycomb bodies in order finally to pass into the reaction chamber. In the reaction chamber, the exhaust gas is further heated by additional heating devices until thermal oxidation of the unwanted exhaust gas constituents can take place. The exhaust gas then flows through a further heat storage means to the exhaust pipe and is discharged into the environment.
In operation, the flow direction is alternately altered whereby the exhaust gas is pre-heated before reaching the reaction chamber, thereby achieving an energy savings in further heating of the exhaust gas. The additional heating effect can be implemented by gas injection or burners (so-called support gas) or an electrical additional heating device. The reaction chamber generally has a free flow cross-section whereby the residence time of the exhaust gas in the reaction chamber is increased and oxidation can take place in the form of a gaseous phase reaction. Carbon monoxide (CO) and methane (CH4) are particularly relevant among the species to be oxidized in the exhaust gas. Such an arrangement is known for example by the trade name CL.AIR® from GE Jenbacher. In that method, exhaust gas is heated to about 700-800° C. and oxidation of the unburnt hydrocarbons and the carbon monoxide is effected to give water vapor and carbon dioxide. The CL.AIR® thermoreactor is in the form of a regenerative heat exchanger and comprises two storage masses, a reaction chamber and a switching-over mechanism. The exhaust gas flows coming from the engine at a temperature of about 530° C. by way of a switching-over mechanism into a first storage mass where it is heated to approximately 800° C. In the reaction chamber, the exhaust gas reacts with the oxygen present, in which case carbon monoxide and unburnt hydrocarbons are oxidized to give carbon dioxide and water. When flowing through the second storage mass, the exhaust gas again gives off heat and is at a temperature of between 550 and 570° C. when reaching the switching-over mechanism which passes it to the chimney or a downstream-disposed waste heat recovery operation.
Regenerative thermal oxidation affords a robust method with which even large exhaust gas mass flows can be economically post-treated.
Thermoreactors as described hitherto are adapted to oxidize both methane and also carbon monoxide. That entails some disadvantages in operation.
In order to be able to break down carbon monoxide, a relatively high temperature and a relative long residence time are required in the thermoreactor.